Notch is required for directing the endocrine/exocrine decision in early pancreatic organogenesis, yet there are large gaps in our knowledge of the involvement of this pathway in later stages of endocrine development. Conditional removal of Presenilin (required for Notch activation) using Ngn3-Cre coupled with lineage tracing of deleted cells revealed that Presenilin-deficient islet progenitors are diverted from the endocrine towards an acinar-like fate. We propose mechanistic experiments that will define the molecular requirements downstream of Presenilin to determine if ?-secretase activity and Notch signaling are involved. Additionally, we ask whether Presenilin is only required in development or also during [unreadable]-cell renewal in the adult. Specific Aim 1 will define the molecular events mediated by Presenilins that are required for Ngn3 expressing, committed endocrine precursors to progress to mature [unreadable]-cells. Notch involvement will be tested using a ?- secretase inhibitor with lineage tracing on embryonic rudiments in vitro. Additionally we will ask if cells deficient in RBPj?, a DNA binding protein that mediates signaling by all four Notch receptors, acquire endocrine fates by crossing the Ngn3-Cre with the RBPJ? conditional allele (R. Kopan) with a Z/EG reporter (Ngn3-Cre; RBPj?c/c; Z/EG). If ?-secretase activity of the Presenilins is not responsible for the fate switch, we will assess the involvement of alternate Presenilin functions. Aim 2 will position the Presenilin-dependent step within the endocrine program by removing Presenilin with Pax6- or Insulin-driven Cre on a Z/EG background. One possible outcome would be a separation of the differentiation and proliferation defects resulting in expanded endocrine mass without altering their original endocrine fate. Aim 3 will determine whether Presenilin activity is involved in maintenance and/or regeneration of mature [unreadable]-cells using Inducible deletion models (Ngn3-Cre-ERTM and RIP-Cre-ERTM) on a conditional Presenilin-conditional background in the adult. The results of these experiments will help define the role of Presenilin activity in neogenesis of [unreadable]-cell precursors and in [unreadable]-cell replication. In summary, the current application proposes to understand the molecular basis of a genetic observation: Presenilin-deficient, Ngn3 expressing progenitors are diverted to an acinar-like fate. We will utilize novel genetic models to elucidate the role of Notch signaling in this phenomenon and more importantly, the role of Presenilin in pancreatic organogenesis, maintenance and regeneration. A developmental switch in the formation of pancreatic islets has been uncovered. Experiments are proposed to define the mechanisms. The results of these experiments may serve to accelerate efforts towards the development of cell-based therapies for insulin delivery. [unreadable] [unreadable] [unreadable]